Wavelength selective optical switches can be used, for example, in routing nodes of a wavelength division multiplex network, i.e., a network in which information associated with to various communications or groups of communications modulates separate carriers spaced apart by tens of nanometers to tenths of nanometer, depending on the requirements and the technologies used. For such applications, wavelength selectivity is necessary, so that the various carriers can be routed along different paths. Independence from polarization is also desirable, so that polarization state control systems need not be introduced and attenuation will not vary with the polarization state. On the other hand, the switching speed need not be very high, response times of the order of some milliseconds being sufficient.
A polarization independent switching element particularly suited for low switching speed applications is described by A. C. Boucouvalas and G. Georgiou in the paper "Fibre-optic interferometric tunable switch using the thermo-optic effect", Electronics Letters, Vol. 21, No. 11, May 23, 1985. The switch consists of an optical fiber Mach- Zehnder interferometer, in which the optical length of one of the two branches can be varied through a heating electrode applied to a thin film connected to the fiber in the branch. By applying adequate voltages to the electrode, the power associated with an optical input signal is split as desired between the two outputs.
Thermo-optical switches based on the same principle can be used to form switching matrices with several inputs and outputs, as described in the paper "Silica-based optical- matrix switch with intersecting Mach-Zehnder waveguides for larger fabrication tolerances" by M. Kawachi et al, Conference OFC/IOOC '93, Feb. 21-26, 1993, San Jose, Calif, (U.S.A.), paper TuH4.
The behavior of thermo-optical switches, however, is substantially independent of wavelength. Therefore, they must be coupled to optical bandpass filters for wavelength selective routing.
The lack of selectivity is also typical of waveguide switches using electro-optical effects or semiconductor amplifiers as optical ports; moreover, such switches are markedly polarization dependent.